As one skilled in the gas turbine engine's turbine blade technology appreciates, the formation of the internal cooling passages in a turbine blade present formidable problems. Typically, these passages are formed in the molding process for casting the blade utilizing ceramic cores. These ceramic cores are extremely brittle and the thickness is finite and obviously, have the propensity of breaking. Because of the material and the finite sizes of these passages the producibility of the blades are adversely impacted. Not only is the blade designer confronted with these molding problems, he must also take into consideration the effectiveness of cooling of the blades by virtue of these cooling passages. This coupled with the fact that the sizes and locations of the passages internally of the blade are critical, the cooling requirements of the blades in order to maintain its structural integrity in the engine's hostile environment are severe compounds the complexity of the designer's task. In addition the designer must be cognizant of the fact that the air utilized for cooling is taken from the engine after a certain amount of processing of this air has occurred which obviously impacts the performance of the engine. Not only is the amount of work that has been done on that cooling air represents a penalty to the engine's performance, it is obviously necessary to utilize only that much air that will handle the cooling load. Any extra air used for this purpose accounts for an additional deficit in the performance of the engine. Hence, it is abundantly important that the blade designer designs the cooling aspects of the blade so that only the exact amount of cooling air is utilized over the operating envelope of the engine. Also, the designer must assure that the integrity of the blades is not compromised while at the same time the efficiency of the engine is not jeopardized.
This invention solves the problem of effective blade cooling while at the same time designing the core used in molding the blade such that the blade structure is enhanced while the breakage of the core used in forming the cooling passages is eliminated or minimized. As is well known in this industry, core breakage has been a problem confronting the blade designer for some time. As will be described in more detail hereinbelow, by virtue of this invention both breakage and distortions of the end part and the ceramic core assembly are eliminated or minimized. The core of the turbine blade is formed from a unique configuration which significantly enhances the producibility of both the end part and ceramic core that is utilized to make the end part. The turbine blade fabricated from this invention involves a 4-walled, cooled, single-piece configuration and utilize a three piece assembled core in which each piece is made using production-oriented methods and materials. The blade to which is being referred herein is not to be mistaken with the heretofore known multi-piece cores and 4-walled castings.
In accordance with this invention, the turbine blade is formed from a unique configuration in which individual segments which make up the inner walls of the 4-walled casting taper and merge, alternating from either side of the airfoil while fairing radially inward, until they line up to form a single wall, parallel to and centered in the blade attachment portion. Essentially, this configuration effectively results in a 4-walled airfoil blending into a 3-walled attachment region with each cavity element maintaining direct radial flow coolant feed.
This configuration for producing the ceramic cores necessary in making the hollow castings affords several advantages. For example, in a 3-piece type core, where the ceramic core elements which produce the hybrid cavities are made separately from the main body ceramic core element, the present invention allows the hybrid cores on both sides of the main body core to be substantially thickened in the blade attachment area without excessively displacing the main body core. This is a benefit because the ceramic cores which produce the hybrid cavities are cantilevered, from the attachment area, for the entire length of the airfoil and would be too susceptible to breakage or deflection without thickening. Another benefit from thickening the hybrid cores in the attachment area is that coolant flow entry losses are minimized by having a larger feed area directly in line with the coolant flow.
The problem associated with the molding process is perhaps best understood by referring to FIG. 1 which is a prior art illustration of the formation of the passageways in the turbine blade. As noted in FIG. 1, the attachment portion of the blade depicted schematically is indicated by reference numeral 110. The inner passages formed by the multi-piece core is depicted by reference numeral 112 and location of the core joints are depicted by the dash lines 114. The disadvantage of this prior art configuration are numerous and include, without limitation, the following:
a) these hybrid cores can not be thickened on both sides without displacing the main body; PA1 b) On a 3-piece assembled core configuration, the core joints would require a matched plug and socket type of contour, increasing the potential for mismatched surfaces; PA1 4) Flash at the core joints would occur perpendicular to the coolant flow causing blockage of the cooling air; PA1 5) Direct access for flash removal is difficult, if not impossible; Complete flash removal causes risk of damaging the surfaces of the blade neck; and PA1 6) There are no provisions for selectively metering coolant flow into a particular hybrid cavity.
Not only does this invention afford advantages in producing the ceramic cores necessary in making the hollow casting, it affords advantages to the cast part. As one skilled in this art appreciates, when a multi-piece ceramic core is used to produce a hollow casting, metal finning or flash can occur at the joint lines of adjacent core elements when the gaps between the core element mating surfaces are not completely filled by the bonding material. This invention obviates or minimizes this problem because the core element joints are positioned in a location that is parallel to and centered in the blade attachment portion therefore providing direct access for flash removal through the attachment coolant feed passages. Since any remaining flash or partially removed flash will be parallel to the coolant flow stream, this remaining flash will not interfere with the coolant flow stream and hence, flow blockage will not occur.
This invention also lends itself to attaching a metering plate at the foot of the blade so that the cooling air can be selectively metered into a hybrid cavity. This is particularly important where it is desired to improve the film cooling effectiveness across the outside wall of the blade. The metering plate allows the designer to size the inlet to the internal cavities so that the pressure supplied therein is controlled so as to provide a desired pressure ratio between the coolant and the engine's working medium or gas path.